1. Field of Invention
The invention relates generally to a control device and control method for a powertrain, a program for implementing the control method, and a recording medium that contains the program, and, more specifically, to a technology for controlling a powertrain that includes a drive power source which transfers torque to an output shaft connected to wheels, and a rotary electric machine which transfers torque to the output shaft via a transmission.
2. Description of Related Art
Hybrid vehicles, in which an engine and a rotary electric machine, for example, a motor are used in combination as drive power sources, have been conventionally used. The torque output from the rotary electric machine is used to move the vehicle or to assist the engine. In some hybrid vehicles, the torque output from a rotary electric machine is transferred to wheels via a transmission that provides several sets of gears to produce the necessary gear ratios.
However, the transfer of torque from the rotary electric machine to the wheels via the transmission may be temporarily interrupted when the transmission is shifted to a selected gear. Therefore, a hybrid vehicle that is structured in a manner such that torque is continuously transferred to wheels has been suggested.
Japanese Patent Application Publication No. 2002-225578 (JP-A-2002-225578) describes a hybrid vehicle in which at least part of power transfer paths, through which powers from multiple drive power sources are transferred to wheels, is made common, and a power transfer state control device (transmission), which changes the state of power transfer between two rotational members, is provided in the power transfer path through which the power output from a specific drive power source (rotary electric machine) from among the multiple drive power sources is transferred to the wheels. The power transfer state control device is structured to change at least one of the rotational speed ratio between the two rotational members and the power transfer path between the two rotational members.
In the hybrid vehicle described in JP-A-2005-225578, even when the state of power transfer between the two rotational members is changed while the power from the specific drive power source is transferred to the wheels, the power from the drive power source other than the specific drive power source is transferred to the wheels. As a result, it is possible to suppress a decrease in the torque transferred to the wheels.
In a hybrid vehicle, a regenerative braking operation, in which a rotary electric machine is operated as a generator and electric energy is collected, is usually performed when the vehicle is decelerating. However, if the SOC (state of charge) of, for example, a battery is high, the generated electric power cannot be stored in the battery. Therefore, the electric power generation performed using the rotary electric machine, i.e., the regenerative braking operation, is restricted. When the electric power generation performed using the rotary electric machine is restricted, for example, a fuel-supply cutoff operation is performed in an engine in order to apply a sufficient amount of braking force to the vehicle. In a vehicle provided with a transmission between a rotary electric machine and an output shaft as in the hybrid vehicle described in JP-A-2005-225578, if gears are changed in the transmission and the rotational speed of an output shaft of the rotary electric machine is decreased in the above-described state, inertia torque of the rotary electric machine may be applied to the output shaft. At this time, a backlash in, for example, a differential gear unit provided between the transmission and wheels is reduced (the amount of a clearance between gears is reduced). As a result, the gears may contact each other, which causes a shock.